I just upgraded from an old Athlon 1.7 to a new core2duo 6300 and an Asus P5V-VM DH motherboard.
After a lot of trouble now the xp pro work without any errors, while i kept my boot harddisk with everything. BUT while in the device manager i see two cpus (and in the boot it reports dual core), in the task manager and in cpu-z i see only one core.
TheGingerOne is right on, switching to the MultiProcessor HAL will allow your OS to use two cores but I need to add that complete XP reinstall may be needed in order to eliminate any underlying issues, after all, you switched to a new motherboard and CPU while keeping your old OS, expect problems down the road...
I would say it sounds like you should do a clean install so that you don't run into any issues down the line related to this. It's not a recommended procedure to upgrade your mobo/cpu and keep "the boot harddisk with everything". (I'm assuming that means that you tried to keep the same install of XP with new components.)
When upgrading you should just find a cheap external HDD to dump your files to before formatting your primary partition.